Treatment of wastewater, especially wastewater containing organic matter, is usually carried out using biological processes involving contact with a biomass (living microorganisms) maintained in suspension or fixed on a supporting media. These processes are usually oriented toward treatment of wastewater from large cities and/or industries. They are conceived to work with minimum flows and loads corresponding to more than one hundred persons. Some people have been trying to adapt it to smaller installations without too much success.
Prior art methods involving the use of a suspended biomass are mainly based on a substrate/biomass ratio which is affected by flow and load variations. These methods aim at reaching an equilibrium state and require frequent adjustments to maintain a stable efficiency. In general, these processes consist in mixing and aerating the microorganisms and the incoming wastewater for a certain period of time before transferring the wastewater to a secondary clarifier for the solid-liquid separation. Furthermore, in order to keep the substrate/biomass ratio as constant as possible, additional equipments (pumps, valves, piping, etc.) are required to transfer the biomass from the secondary clarifier to the biological reactor. Also, aeration control within a certain range is a critical factor. Flow variations also have negative effects on the standard secondary clarifier by varying the water upflow velocity that could carry suspended solids over the effluent weir. Apparatuses and methods involving a suspended biomass are described by way of examples in U.S. Pat. Nos. 3,666,106; 3,767,051; 3,966,604; 44,104,167; 4,608,157; 4,663,044; 4,664,795; 5,162,093; 5,268,094 and German Patent No. 0,294,682.
Prior art methods involving the use of a fixed biomass can be performed in different ways. One of these ways involves the use of a coarse media (trickling filter) with a fixed or moving water distribution system, with or without water recirculation (additional pumps or not). According to another one of these methods, fine particles media (bio-filtration, peat moss filtration, etc.) may be used with a fixed or moving water distribution system and, furthermore, with a filter backwash system. According to another one of these methods, moving media (biodisks) can be used either partially or fully submerged in relatively stagnant water or not. According to this last method, a flow and/or load variation, because of the constant metabolism of the microorganisms and the impossibility to vary their quantity rapidly in the unit, results in an efficiency drop. The only possible intervention is the modification of the recirculation rate when this option is available. A human intervention is then required to analyze pollutant parameters and make the necessary adjustments but this cannot logically be done for daily variations.
When the preceding treatment processes are used for small flows and weak loads, because of relatively high variations, the main adjustments (air, mixing, recirculation etc.) must be made under average conditions. However, the unit will never operate exactly at average flow and effluent quality variations that could occur could be unacceptable. Complex and expensive control systems have been used to maintain efficiency as stable as possible. This is not economically justifiable for small units.
In the majority of cases belonging to the prior art, clarifiers are used before and after the biological reactor to remove all settleable solids before entering the reactor or before discharging into the environment. These units are designed to operate with continuous flows and are very sensitive to flow variations. Indeed, changes in the water velocity induce turbulence that may cause the light settled solids to return in suspension. The total unit efficiency relies greatly on this ability to retain solids.
In the case of small flows, certain elements (pipe diameter, pumps, etc.) are overdesigned to minimize clogging problems. This can cause low velocity flow in areas where solids settling or high flows in other places would be undesirable.
To avoid prior art disadvantages related to mechanized treatment units, it has been already suggested to use aerated or non-aerated lagoons for medium flows and weak to medium loads. This type of technology does not allow any control since efficiency depends solely on the temperature and the hydraulic retention time which is linked to the basins volume and inlet flow. This alternative occupies a relatively important land area which shall be as horizontal as possible, with exposed water surfaces that could be undesirable near dwellings. Hydraulic retention time is measured in days. A minimal distance should be kept to the nearest house to avoid odor detection for particular climatic conditions. For smaller flows, septic tanks followed by buried weeping tile fields have been used and continue to be used mainly in rural areas. Weeping tile fields are subject to clogging especially when built in non-permeable soils. It requires a relatively big area and shall be built at a minimum distance from any drinking water source. After construction, the tile field is not readily accessible for inspection and/or modifications.
Also known in prior art, there is the trickle filtration apparatus described in UK patent application No. 2,239,237-A. This apparatus is designed to be sunk into the ground, leaving only an upper portion of the apparatus above ground level. It comprises a primary settlement tank side-by-side with a secondary settlement tank. A pump in a chamber pumps supernatant from tank which has entered chamber through inlet continuously to the top of filter media in a filter tank above the first mentioned two tanks. Supernatant trickles down the filter media and is collected at the bottom to flow through a conduit back to the pump chamber. Inflow of sewage into the primary tank results in supernatant spilling out of the conduit at a break point into the secondary settlement tank and in treated liquor spilling out of the latter through an outlet. Oxygenation of the biomass on the filter media is by natural air convection. A drawback with this type of apparatus is that, because of its design, the content of the primary tank rapidly passes in an anaerobic state. This is due to the fact that there is no recirculation of water in this primary tank. As well known in the art, anaerobic degradation is slow and causes formation of foul gases.
In short, actual wastewater treatment units are either complicated to operate, sensitive to hydraulic shocks or inefficient. They are generally of a large size and have to be operated by skilled workmen.
Therefore, there is a very strong need for a wastewater treatment unit, especially a wastewater treatment unit for wastewater having an organic content, capable of handling efficaciously effluents from different sources including those from single family homes, and remaining operable even under high intermittent flow rate variations without constant human interventions.